SANTA CLARA -- Before Eric Reid headed out of the locker-room tunnel for his successful 49ers debut Sunday, defensive tackle Justin Smith gave him some sage advice.

"Stay back. Stay the heck back today. We'll take care of everything up front," Smith told Reid, according to coach Jim Harbaugh, who followed them out of Candlestick Park's tunnel.

Reid, the 49ers' first-round draft pick, didn't exactly stay back and out of the fray. He stepped up for a positive Game 1, stepping into the starting free-safety role Dashon Goldson previously occupied since 2009.

One snap after he a missed tackle, Reid made an interception, and he finished with six tackles, several hard hits and even special-teams contributions in the 49ers' 34-28, season-opening win over the Green Bay Packers.

"He's been really diligent, on it, from the time he got here. That's the way he's wired," Harbaugh said. "He's not an error repeater. He takes great pride in his own performance. He's really smart. He has a lot of good qualities.

"Ever since he's been here, he's picked things up extremely fast. It's early on, and we're not going to have to worry about Eric Reid picking up the defense, where his alignment should be, what his assignments are. He does a great job."

Reid became the first 49ers rookie to intercept a pass on opening day since Don Griffin and Tim McKyer each did so Sept. 7, 1986 at Tampa Bay. Reid's came after Jermichael Finley failed to catch Aaron Rodgers' second-quarter pass at the 49ers' 44.

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"I don't know what I was thinking but I wish I kept the ball," Reid said. "It was one of those covers where I read the quarterback and I broke on the throw to the receiver. And the ball fell right into my hands."

Reid snagged another Rodgers' pass, and although this one at the 49ers' goal line didn't count as an interception because of an offside penalty on Aldon Smith. Reid beat the Packers' Randall Cobb to the ball, and had Cobb caught it, a touchdown would have stood and the penalty declined.

Harbaugh praised that play, adding of Reid: "I have no doubt he belongs and he's going to get better and better as we go."